1. Field of the invention
The present invention concerns a fiber-optics connected optoelectronic device mounted on a substrate. It finds an application in the manufacture of an optical receiver for a fiber-optics transmission system.
2. Description of the prior art
In a system of this kind a light wave carries a digital signal which is either intensity modulated and directly detected or frequency or phase modulated and coherently detected. In the case of a high bit rate, signal detection relies on devices operating in the microwave frequency range (1.8 GHz, for example) and these devices are advantageously integrated into a hybrid circuit on a ceramic substrate.
In a receiver of this kind an input light signal is guided by an optical fiber onto a photodiode connected to a first amplifier of the receiver. The electrical interconnection of these two devices is a difficult technical problem when the frequency of the electrical signal is high: the spurious capacitance of the connection line imposes a limitation on the bandwidth of the receiver and introduces additional noise. Furthermore, if the distance to the photodiode is increased the impedance mismatch between the photodiode and the amplifier causes standing waves to appear on the line and these disrupt the operation of the circuit. Various proposals have been made for solving this problem; they all involve eliminating the casing in which the photodiode is conventionally contained and providing a direct electrical connection from the photodiode to the amplifier in order to eliminate the spurious capacitance of the casing. Holding the optical fiber in a position ensuring a good optical connection then becomes a problem and is made more complex if the device must be designed to withstand mechanical stresses. One solution to this optical connection problem is described in the document WO-A-87/01465 - BRITISH TELECOMMUNICATIONS (which corresponds to U.S. Pat. No. 4,830,450). The optical fiber 4 is held directly by the edges of an optical connection hole formed for this purpose in a ceramic substrate 1 using a laser. However, according to this document a hole drilled by a laser in a ceramic substrate has a conical shape which makes it impossible to align the optical fiber correctly. For this reason the substrate is made by stacking a plurality of ceramic platelets through each of which a conical hole has been drilled. These holes are aligned and the cones are oriented in the direction which facilitates insertion of the fiber. This embodiment of an optical connection is complex and costly.
The present invention is directed to solving these connection problems in a simpler way.
Although consideration is given above to problems associated with implementing an optical receiver by means of a photodiode, it must be understood that the present invention also applies to the implementation of devices other than an optical receiver of this kind and to the connection of optoelectronic components other than a photodiode of this kind, for example a phototransistor, an optically controlled microwave component or a semiconductor laser, when these components are carried by ceramic substrates.
In all these various situations a general objective of the invention is the simple implementation of a durably accurate optical connection on a ceramic substrate such as that of a hybrid circuit.